Optical components, such as lenses, used in optical pickups and other optical devices are conventionally made of glass, plastic, a single crystal of lithium niobate (LiNbO3), or the like, as disclosed in, for example, Patent Documents 1 and 2.
Glass and plastics have high light transmittances and are easy to form into a desired shape. Accordingly, those materials are mainly used for optical components such as lenses. LiNbO3 single crystal is mainly used for optical components such as optical waveguides on the basis of the electro-optical characteristics and double refraction of the LiNbO3 single crystal.
Optical pickups and other optical devices including such optical components are required to be further downsized and to be thinner. Unfortunately, the refractive indices of conventionally used glass and plastics are 2.00 or less. It is therefore difficult to downsize the optical components and optical devices using such a material and to reduce their thicknesses. In addition, plastics disadvantageously have low moisture resistance, and besides may cause double refraction. It is accordingly difficult for plastics to transmit and concentrate incident light efficiently.
On the other hand, the LiNbO3 single crystal has a high refractive index of 2.3, but may cause double refraction. LiNbO3 single crystal is thus unsuitable for optical components such as lenses, and is limited in application.
For example, Patent Document 3 discloses Ba(Mg,Ta)O3 and Ba(Zn,Ta)O3 translucent ceramics as materials that causes no double refraction and provide superior optical characteristics. These ceramics have refractive indices (hereinafter referred to as refractive indices at a wavelength of 633 nm, unless otherwise specified) of 2.01 or more.
It may be desired that the anomalous dispersion Δθg,F, which is one of the optical characteristics, be high. To have an anomalous dispersion means to have wavelength dispersibility different from normal optical glass, as will be specifically described below. A high anomalous dispersion Δθg,F is advantageous in correcting chromatic aberration. The anomalous dispersion is represented by a negative number, and a high anomalous dispersion means that its absolute value is high.
Patent Document 3 discloses Ba(Mg,Ta)O3 and Ba(Zn,Ta)O3 translucent ceramics having perovskite structures represented by the general formula ABO3. In particular, the B site of the structure is constituted of at least two elements, and these translucent ceramics have complex perovskite structures. More specifically, the electroneutrality can be substantially maintained by controlling the molar ratio of divalent metallic element(s) mainly constituted of Mg and/or Zn to the pentavalent metallic element(s) of Ta and/or Nb to about 1:2. In addition, the optical characteristics, such as refractive index and Abbe number, can be changed by substituting a tetravalent element, such as Sn or Zr, for the B site elements Mg, Ta, and/or Zn.
However, the translucent ceramics disclosed in Patent Document 3 disadvantageously have a low anomalous dispersion Δθg,F. For example, a Ba{(Sn,Zr)Mg,Ta}O3 ceramic has a Δθg,F value of −0.013; a Ba(Zr,Zn,Ta)O3 ceramic has a Δθg,F value of −0.006; a Ba{(Sn,Zr)Mg,Nb}O3 ceramic has a Δθg,F of −0.000.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-127078 (all pages, FIG. 1)    Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-244865 (Claim 6, paragraph 0024)    Patent Document 3: International Publication No. 02/49984 pamphlet (all pages, all figures)